The specification and morphogenesis of skeletal muscle is critical for normal embryonic development. The critical role of muscle is highlighted by the fact that congenital muscular dystrophies cause debilitating disease in children and are associated with premature death in nearly all patients affected. Many congenital muscular dystrophies, such as Duchenne, Ulrich, and Merosin-deficient muscular dystrophies, are caused by mutations in adhesion complexes that anchor muscle cells to their surrounding extracellular matrix (the myomatrix). In addition to being required for muscle development, the myomatrix plays a fundamental role in muscle homeostasis because it bears much of the passive load. Despite the critical role that the myomatrix plays in muscle development and physiology, the mechanisms underlying myomatrix development are not known. In particular, mechanisms that mediate the dynamic changes in myomatrix composition are not well understood. Fibronectin (Fn) is a myomatrix protein that is necessary for muscle development and regeneration. Fn is transiently upregulated during muscle development and regeneration. The subsequent downregulation of Fn levels is important because excess Fn deposition leads to fibrosis. Fibrosis is the aberrant deposition of extracellular matrix proteins. In the context of skeletal muscle, the replacement of contractile muscle tissue with Fn-rich fibrotic material diminishes muscle function. Therefore, Fn levels have to be tightly controlled. The mechanisms that mediate Fn polymerization during muscle development are beginning to be elucidated, but nothing is known about how Fn is downregulated. Our central hypothesis is that the myomatrix protein laminin, along with Matrix Metalloproteinase 11 (Mmp11) and Tissue Inhibitor of Matrix Metalloproteinase 2a (Timp2a), comprise a regulatory network that controls Fn levels, muscle fiber type specification, and muscle morphogenesis. This hypothesis is based on our preliminary data showing that laminin polymerization mediates Mmp11 localization and that Mmp11 is necessary and sufficient for Fn downregulation during zebrafish development. Preliminary data also suggest that Timp2a is required for Fn regulation. Understanding Fn regulation during muscle development and regeneration is significant because transient Fn is necessary; but too much or sustained Fn is deleterious. These experiments are innovative because they are the first to address mechanisms underlying Fn downregulation during development, the first to address Mmp11 function during muscle development, and the first to analyze crosstalk between laminin and Fn. The contribution of this research will be identification of novel mechanisms regulating Fn levels and roles for Fn in muscle fiber type specification. Completion of this research will significantly advance our long-term goal, which is to understand how signaling between muscle cells and their myomatrix mediates muscle development, homeostasis, and regeneration.